Field of the Invention
The present invention relates to an array substrate. More particularly, the present invention relates to an array substrate including a thin film transistor with an oxide semiconductor layer in which reliability of the thin film transistor is obtained by restraining characteristics of the thin film transistor from varying due to light incident on the substrate.
Discussion of the Related Art
With rapid development of information technologies, display devices for displaying a large amount of information have been developed. More particularly, flat panel display (FPD) devices having a thin profile, light weight and low power consumption such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, electroluminescence display (ELD) devices, and organic light emitting diode display (OLED) devices, which may be referred to as an organic electroluminescent display devices, have been actively pursued to replace the cathode ray tubes (CRTs).
Among these devices, liquid crystal display devices have been widely used for monitors for notebook computers and personal computers or televisions because of their high contrast ratio, superiority in displaying moving images, and low power consumption. The performance of liquid crystal display devises is based upon optical anisotrophy and polarization properties of liquid crystal molecules with the device. Liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules.
In addition, organic electroluminescent display devices have been recently drawn interest because of their many beneficial characteristics such as high brightness and low driving voltages. Because they are self-luminous, the organic electroluminescent display devices have excellent contrast ratios and ultra thin thicknesses. The organic electroluminescent display devices have a response time of several micro seconds, and have an advantage in displaying moving images. Also, the organic electroluminescent display devices have wide viewing angles and are stable under low temperatures. Since the organic electroluminescent display devices are driven by a low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits; and the manufacturing processes of the organic electroluminescent display device are simple since only deposition and encapsulation steps are required.
Each of the liquid crystal display devices and the organic electroluminescent display devices includes an array substrate having thin film transistors as switching elements to control their respective pixels.
The array substrate also includes gate and data lines crossing each other to define pixel regions, and at least one or two thin film transistors functioning as a switching or driving element are formed in each pixel region.
The thin film transistor has various structures depending on materials for a semiconductor layer.
Namely, the semiconductor layer may be formed of amorphous silicon, an oxide semiconductor material, or polycrystalline silicon, and the thin film transistor may have a top gate or a bottom gate structure due to the material for the semiconductor layer.
Recently, an array substrate having a thin film transistor with an oxide semiconductor layer of an oxide semiconductor material has attracted attention.
A thin film transistor having an oxide semiconductor layer has superior conductivity of carriers as compared with a thin film transistor having a semiconductor layer of amorphous silicon, and its fabrication is simplified in comparison with a thin film transistor having a semiconductor layer of polycrystalline silicon because the thin film transistor having an oxide semiconductor layer does not need processes such as doping impurities.
FIG. 1A is a schematic plan view illustrating an array substrate having a thin film transistor with an oxide semiconductor layer according to the related art.
In FIG. 1A, the related art array substrate 1 has a thin film transistor Tr, and the thin film transistor Tr includes an oxide semiconductor layer 20, a gate insulating layer (not shown), a gate electrode 15, an inter-insulating layer (not shown) having semiconductor layer contact holes 23 and 24, and a source electrode 26 and a drain electrode 29 spaced apart from each other. The source and drain electrodes 26 and 29 contact the oxide semiconductor layer 20 through the semiconductor layer contact holes 23 and 24, respectively.
The thin film transistor Tr including the oxide semiconductor layer 20 is sensitive to light. Therefore, characteristics of the thin film transistor Tr including the oxide semiconductor layer 20 are changed due to light incident on the oxide semiconductor layer 20 and driving reliability of the thin film transistor Tr including the oxide semiconductor layer 20 is lowered for each location based upon the incident light.
To solve the problem, a structure for screening the light incident on the oxide semiconductor layer 20 has been introduced.
That is, a light-shielding pattern 10 with an area corresponding to the thin film transistor Tr, which is floating, is formed under the oxide semiconductor layer 20 to screen the light incident on the oxide semiconductor layer 20.
However, the light-shielding pattern 10 is differently charged for each location, and an undesirable channel is caused in the oxide semiconductor layer 20. Thus, the characteristics of the thin film transistor Tr are lowered, and image qualities of a display device decrease.
Namely, in a graph showing current-voltage characteristics of the thin film transistor Tr, as shown in FIG. 1B, there is a difference between effective gate voltages due to a difference in quantities of electric charges charged in the light-shielding pattern 10, and this causes a difference in drain currents. Therefore, current-voltage curves are shifted, degrees of the shifts of the curves are varied according to drain voltages, and the threshold voltage of the thin film transistor Tr is changed. Accordingly, the characteristics and reliability of the thin film transistor Tr are lowered.